Development of a high-speed and large capacity optical transmission is progressing, and WDM transmission attracts attention in this regard. Moreover, in the WDM transmission, a new problem of wave distortion of the signal light because of a nonlinear phenomenon occurs. It is known that it will be easy to generate a nonlinear phenomenon, for instance, Self-Phase Modulation (SPM) and Cross-Phase Modulation (XPM), etc, by higher power of the input signal light to the optical fiber.
In general, this wave distortion A NL by SPM or XPM is shown by the following expression.λNL=(2π·n2·Leff·P)/(λ·Aeff)                (Here, n2 indicates a nonlinear refractive index, Leff indicates an effective length, P indicates an input optical power, λindicates a wavelength, and Aeff indicates an effective area of the optical fiber.)        
Therefore, in order to suppress the occurrence of the nonlinear phenomenon such as SPM or XPM, it is effective to use the optical fiber that has large Aeff.
Single mode optical fiber (SMF), that has zero dispersion wavelength in 1.31 μm band, has a larger Aeff of about 80 μm2 at a wavelength of 1550 nm than that of a Dispersion Shifted Fiber (DSF) that has zero dispersion wavelength in 1.55 μm band. Therefore, the nonlinear phenomenon such as SPM or XPM doesn't appear easily when SMF is used for optical transmission in 1.55 μm band. (1.55 μm band corresponds to the wavelength band of 1530-1570 nm, hereinafter.)
Moreover, because SMF have a large chromatic dispersion in the 1.55 μm band (for instance, the dispersion is 17 to 22 ps/nm/km at a wavelength of 1550 nm), it is easy to avoid Four Wave Mixing (FWM), the other nonlinear phenomena.
In addition, it provides with the advantage such as low loss and low Polarization Mode Dispersion (PMD).
On the other hand, since the dispersion is too large, the problem of wave distortion of signal light by the cumulative dispersion occurs, and which causes big hurdle in the optical transmission.
This problem can be solved by compensating the dispersion of SMF by using the negative dispersion optical fiber, which has a negative chromatic dispersion at a wavelength of 1550 nm.
For instance, U.S. Pat. No. 6,421,489 discloses the optical transmission system using a SMF as a transmission line and compensating the dispersion by a Dispersion Compensating Fiber (DCF) that has a large absolute value of a negative chromatic dispersion. This DCF is usually used in a form that is rolled like the coil and stored in a compact package, so-called the module, and is set up in the relay station etc.
Moreover, U.S. Pat. No. 6,456,770 discloses the optical transmission system using a SMF and an Inverse Dispersion Fiber (IDF) which has an almost the same absolute value as SMF and a negative chromatic dispersion, as a transmission line.
These DCF and IDF have very small Aeff compared with SMF, and hence the nonlinear phenomena appear easily, and also have the disadvantage of high loss and high PMD.